1. Field of the Invention
The invention relates generally to free piston machines, and more particularly to a gas bearing apparatus, and a method of making the gas bearing apparatus, for a free piston machine.
2. Description of the Related Art
Pistons in many machines are connected to a rigid, mechanical link, such as a connecting rod connected to a crank shaft. These pistons are confined within predetermined positions, such as end limits. However, many machines are known which use one or more free pistons.
A free piston reciprocates in a cylinder without a mechanical connection. Such free pistons may be driven by an electromagnetic, linear motor and used, for example, as a gas or other fluid compressor or pump Free pistons are also found in free piston Stirling cycle machines, such as free piston Stirling cycle engines, coolers and cryocoolers.
Free pistons sealingly reciprocate in a cylinder formed in a housing, with a very small gap formed between the cylinder wall and the piston wall. The housing typically encloses a work space bounded by one end of the piston and a second space, or back space, bounded by the opposite end of the piston. A working gas, such as helium, fills the workspace, back space and other regions of the machine within the housing.
Because of the close proximity of the piston wall and cylinder wall during operation, the gap formed between the walls must be lubricated to prevent rapid wear. The most effective lubrication has been found to be a thin layer of the working gas forming a gas bearing. Such gas bearings are described in U.S. Pat. Nos. 4,412,418, 4,802,332 and 4,888,950, all to Beale.
In order to lubricate the piston, gas must be directed into the gap at three or more points around the circumference of the piston after being routed from the workspace or back space. However, transporting and releasing the gas into the gap requires a complex network of passages and ports. Such passages and ports are not easily formed, because the parts into which gas-transporting structures must be formed are small, delicate and made to close tolerance.
It is known to form a shrink fit annular valve sleeve assembly as described in U.S. Pat. No. 5,184,643 to Raymond. Such assemblies will not work for the purpose of forming a gas bearing on a free piston machine due to a lack of control over gas pressures, and a lack of passages for directing the gas against a cylinder wall.
Therefore, the need exists for a gas bearing structure, and a method of making the same, for a free piston machine.